Ferrite heater and heat sensor



Sept. 16, 1958 c. F. KOOl FERRITE HEATER AND HEAT SENSOR' Filed Aug. 17; 1956 AMPLIFI AMPLIFIER RECTIFIER MECHANICAL FILTER DISCRIMINATOR FILTER OPERA 771V 6 7' IMP.

INVEN TOR. 62 AREA/CE E Kaa/ flrraR NEY FERRITE HEATER AND HEAT SENSOR f ApplicationAugust 17, 1956, Serial No. 604,834

I scram. or. 219-20 This invention relates to temperature controlled ovens and particularly to ovens that operate at extremely constant temperatures. Ovens of this type are particularly applicable for maintaining quartz oscillator plates at a constant temperature. An oven of the present invention utilizes a single ferrite receptacle to enclose the temperature controlled space, to function as a heater element, and also to function as a heat sensor. The heat sensor includes a Winding that is Wound on the walls of the ferrite receptacle. A change in temperature causes a change in permeability of the ferrite and a resulting change in the inductance of the Winding. Effective temperature control is obtained by using ferrite with a relatively high temperature coefficient of permeability.

An object of the present invention is to provide a reliable, extremely constant temperature oven.

A further object is to provide a temperature-regulated oven that can be manufactured economically.

. A feature'of the oven of the present invention is the utilization of a single ferrite component part for the walls of an oven cavity, for the magnetic core of an oscillator winding which is used as a heat sensor, and for the electric heating element.

A further feature is the ease with which the control system of this oven may be adjusted for obtaining accurately regulated temperature.

Further objects and features will be more readily understood by reading the following description with reference to the drawings, in which,

Figure 1 is a graph that shows change in permeability with change in temperature for a typical sample of ferrite, and

Figure 2 is a schematic diagram, with certain usual circuits in block form, showing-the oven of this invention and its temperature control circuits.

Briefly, the circuit of Figure 2 shows an insulated ferrite container and its control circuit for maintaining a constant temperature within the cavity surrounded by the ferrite Wall. The cavity may be used for maintaining an object, for example, a quartz oscillator plate, at an even temperature indefinitely. A Winding on the wall of the container is connected to function as a frequency determining element in a resonant circuit of an oscillator.

Any temperature change within the ferrite container changes the permeability of the ferrite wall and thereby changes the inductance of the oscillator winding. Obviously, a change in inductance results in a change in frequency of the oscillator output signal. The oscillator is connected to a control circuit which includes a discriminator, a rectifier, and a current control tube connected in cascade. This control circuit develops a current, the intensity of which is dependent upon the frequency of the oscillator. The ferrite Wall of the oven is the heating element and the control current is applied directly thereto. Deviation from a predetermined temperature of the ferrite wall will cause a change in heater current that is applied to the Wall so that the temperature is returned to the predetermined value.

tas Pate through the wall heats the entire oven cavity.

The curve in Figure 1 shows variation of permeability with temperature change for a partic'ularcomposition of ferrite. When an oven with a wall having this composition of ferrite is to be operated at C., a slight rise in temperature over 65 C. causes an increase in permeability of the Wall, or a drop in temperature under 65 C. causes a slight decrease in permeability. This property of changing permeability with temperature change is the basis for the heat sensing element of this invention.

In Figure 2, oven 10 comprises an insulated case 11 in which is placed a ferrite chamber 12. In the particular example shown, the ferrite chamber is in the form of a tube or cylinder. Two electrical connections 13 and 14 are' provided for connecting a heating current circuit to two points spaced apart on the ferrite walls of the container. When the container is a tube, one connection may be made at each end of the tube so that current flow Two notches 15 and 16 are placed oppositely at each end of the tube for receiving a Winding 17 which is the inductor for the oscillator resonant circuit. Since the wall of a cavity, which is both the heating element and the sensor, distributes the applied heat evenly, less insulation 18.is required for lining the surface of case 11 than is normally required for a constant temperature oven of different design. Of course, it is understood that connecting leads for the heater connections and for the oscillator winding pass through the oven case. Feed-through terminals may be used whena hermetic seal is desired.

The ferrite material for the oven cavity must be selected to obtain somewhat different characteristic values than those normally required for cores of inductors that are used in high-Q, high-frequency circuits. Contrary to the requirements for cores of sharply tuned high-frequency circuits, the resistivity of the ferrite should be low for otherwise a very high voltage Would be required to obtain sufiicient heater current. However, the resistivity of the ferrite may be selected from a wide range depending upon the type of power tube and the supply voltage available. Different supply voltages make it possible to use materials having resistivity between 10 ohm cm. and 100,000 ohm cm. Since the heat sensing circuits depend upon a change of permeability, the temperature coefficient of permeability should be quite high. Ferrite material with a temperature coefficient of approximately 10,000 parts per million per degree centigrade Will provide good operation. If materials with lower temperaturecoefficients are used, additional amplification will be required in the control circuit. The permeability is not particularly critical. However, when the material used has high permeability fewer turns are required for the heat sensing oscillator Winding. The Q of the ferrite must be high enough to permit the oscillator to function properly without using excessively large coupling capacitors. A Q of 10 is very satisfactory; however, a Q as low as 5 may be satisfactory. A composition with the following mole percent has been used with good results: 50% F6203, 30% MnO, and 20% ZnO. The material has a temperature coefiicient of permeability of 8,000 P. P. M./ C., permeability of 2,100 at room temperature, and a Q of 5 at 500 kilocycles. The ferrite portion of the oven may have various configurations; however, a suitable length'of ferrite pipe is readily fabricated and provides good operation.

The oscillator portion of the oven control circuit includes pentode electron tubes 19 and 20 which are connected to the resonant circuit that includes oven winding 17. Fixed capacitors 21 and 22 and adjustable capacitor 23 are connected in parallel with the winding 17 to tune the circuit to a desired predetermined frequency. In order to be able to use readily available parts for the succeeding discriminator circuit, the desired frequency may be the same as that commonly used in the intermediate frequency stages of a superheterodyne radio receiver, for example, 455 kilocycles.- Capacitors 21 and 22 may be of the temperature compensating type to compensate for changes in frequency caused by temperature changes on component parts other than that of oven winding 17. Theresonant circuit and grid resistor 24 are connected betweenthe control grid of tube 159 and ground. The cathodes of tubes 19 and are connected through a common cathode resistor 25 to ground for transferring signal voltage to tube 20. In order to sustain oscillation, signal from the plate of tube 20 is connected through feedback coupling capacitor 26 to the resonant grid circuit of tube 19. Resistor ,27 is the usual plate load resistor for connecting the plate of tube 20 to ground. Resistor 28 and capacitor 29' are the usual voltage dropping resistor and bypass capacitor respectively for the screen grids of the tubes. The oscillator operates in'the welllinown manner except that its frequency changes readily with oven temperature, the frequencychange being due to the construction of the ferrite inductor in the resonant circuit.

The signal generated by the oscillator is applied through capacitor 30 to amplifier circuits 31. The amplifier of conventional design amplifies the signal generated by the oscillator and couples it to a succeeding discriminator.

'When a mechanical filter is used in .the discriminator circuit, the amplifier circuit may comprise an amplifier stage and a cathode follower stage. The cathode follower stage provides the relatively low impedance required for matching the input impedanc of the suc eding metha r a l v A discriminator circuit Which-is connected ;to the cutput circuit of the amplifier must be capa le of detecting slight frequency changes. The mechanical filter 32pmvides narrow-band frequency detection for signal having a frequency range within the steep skirt of its resonant curve. An electromechanical filter suitable forlthis appication is described in vU. S. Patent No. 2,615,98lfissued to M. L. Doelz on October 28, 1952. In order to maintain the operation of :the mechanical filter at a constant frequency so that the frequency of the oscillator signal is within the steep portion of its resonant curve, the temperature :of the .filter .must .be heldwithin a range .of 5 .C. The filter may be mounted Within an auxiliary .oven or it ma y be mounted within the ferrite oven 10 itself.

Amplifier circuits 33, rectifier circuits 34, and smoothing filter circuits 35, whichare connected .betweenthediscriminator and the control tube, are .conventional. For example, the amplifier circuits 33 may consist .of one stage of resistance coupled amplification and a succeeding cathode follower stage. The .low impedance output circuit of the cathode follower may be connected to rectifier circuit 34 which may be a plurality of dry disc rectifiers .connected in a voltage-quadrupler circuit. The output circuit of the rectifier is connected through the usual smoothing filter circuit to the control grid circuit of current control tube.36.

The .current control tube 36 may be the usual radio receiver power amplifier tube-havingat-least a cathode, a control;grid, and aplate. Tube 36 is shown as a pentode tube having plate .37 and screen grid38 connected to a source of .direct current voltage 39. The outputcircuit of filter is connected .to control grid d-tlof tube 36 for applying theretoa direct current voltage that is proportional to the deviation in the oscillator frequency, the deviation beingcaused .by change in temperature of ferrite tube 12. Cathode 41 is connected through adjustable resistor-.42, milliampere meter 43, connection 14, ferrite tube 12,.aud connection 13 to ground. A circuit for adjusting .the control grid bias of tube 36 includes resistor 44 and adjustable resistor 45 which are connected in series between .the source of direct current 139 and the cathode circuit oftube .36. The adjustable of re 4 sistor 45 is connected to the return circuit of control grid 40 so that the bias of the tube may be adjusted for obtaining required cathode current in the heater circuit of ferrite tube 12.

Variations in oven temperature from a predetermined value are sensed by the electronic circuits from regulating the current in the heating circuit. For example when the temperature rises very slightly, the permeability of the ferrite chamber 12 increases slightly and produces a small increase in the inductance of Winding 17. The frequency of the signal generated by the oscillator that includes tubes 19 and 20 is accordingly changed to a lower frequency. The signal is applied from the oscillator through amplifier 31 to the mechanical filter discriminator 32. In this, particular embodiment the discriminator is a sharply tuned mechanical filter that has a steep skirt on its characteristic curve.

When the frequency of the signal that is applied to the discriminator is shifted slightly, the amplitude of the signal at the output of the discriminator changes sharply. As an example, a mechanical filter may be tuned so that the input signal'falls on that skirt which causes an 'increase in output signal when the frequency of the oscillator signal becomes lower. The signal from the output of the discriminator is applied through amplifier 33 to rectifier 351. Direct-current voltage developed in the rectifier circuit is applied through smoothing filter 35 to control grid 49 of tube 36. When the rectifier is polarized for developing a negative voltage, current in the heat circuit, which includes ferrite receptacle 12 and the cathode of tube 36, is reduced. ,A reduction in heating current allows the ferrite receptacle to cool slightly *uritil the oscillator operates at the slightly higher frequency for restoring equilibrium at thepre determined temperature of the ovenl The ferrite oven of this invention may be used to maintain a cavity at a constant temperature for along period/ For example, a quartz oscillator platema y'be housed within the cavity to maintain its temperature at 65 C. with variations not to exceed .01" C. Since the walls of the oven, the oven heater element, and thehcat sensor are-all the same one component, little lag time passes between thechange in temperature at the sensing element and the required change 'in application of heat. Various modifications and changes may be made in the configuration of the oven and associated control circuits as shown in the disclosed embodiment without departing from the spirit and scope of invention as 'defined in' the following claims.

I claim:

1. A temperature controlled oven comprising a cavity having a ferrite wall, a winding wound on said wall, heater connections spaced apart on 'said wall, a source of direct current voltage, current control means connecting said direct current source and said connections, control circuits connected between said winding and said current control means, said control circuits being responsive to a change in said inductance for operating said current control means, said current control means operating to change the current flow through said wall for changing the temperature thereof, and the inductance of said winding changing in value in response -to the change of current flow through said wall.

2. In a temperature controlled oven, a receptacle having a ferrite wall for enclosing a space that is to bemaintained at nearly constant temperature, an oscillator including a'resonant circuit for determining the frequency of signal generated thereby, said resonant circuit including a winding disposed on said wall such thatsaid ,wall completes a magnetic circuit for said winding, a frequency discriminator connectedto said oscillator, first and second electrical connections spaced apart on said wall so that current .flow between said connections through said .wall will causeheating thereof and therebychange the permeability of said wall for changing the frequency ofsaid 5 signal, said discriminator responsive to a change of frequency in the signal of said oscillator for developing a change in direct current voltage, and means responsive to a change in the direct current voltage output of said discriminator for changing the current flow between said connections that are on said ferrite wall.

3. A temperature regulating device comprising, a tubular resistive wall, said wall having a high temperature coefficient of permeability, a winding wound longitudinally about said wall, an electrical connection contacting each end of said wall, a control circuit connected to said winding, said'control circuit developing a control voltage of a value determined by the inductance of said winding, a source of direct current and a current control circuit connected to said wall through said connections, and said current control circuit responsive to the change in said control voltage for changing the current flow through said wall, thereby regulating the temperature of said walls as a function of the change of inductance of said winding, the change of inductance being determined by the change of permeability of said wall with change of temperature.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Duncan et al.: Temperature Responsive Behavior of Ferrimagnetic Resonance in Ferrites located in Wave Guide; Journal of Applied Physics; volume 27, No. 3, March 195 6; pages 209-215. 

